The present invention relates to a wheel-support rolling bearing unit, which is used for rotatably supporting vehicle wheels on suspension devices, and also relates to a method of manufacturing the same.
The vehicle wheels are supported on the suspension devices by the wheel-support rolling bearing unit. FIG. 4 shows a conventional wheel-support rolling bearing unit widely used. The conventional wheel-support rolling bearing unit 1 includes a hub 2, an inner ring 3, an outer ring 4, and a plurality of rolling elements 5. A first flange 6 for supporting the wheel is formed at the outer end of the outer peripheral surface of the hub 2 (the word xe2x80x9couter side or outsidexe2x80x9d means the outer side of the vehicle body as viewed in the widthwise direction in a state that the unit is assembled into the motor vehicle, and the left side in those figures except FIG. 8. The word xe2x80x9cinner side or insidexe2x80x9d means locations closer to the center of the vehicle body as viewed in the widthwise direction, and the right side in those figures except FIG. 8.). A first inner raceway 7 is formed in the outer surface of a middle portion of the hub 2. A stepped part 8 having the reduced outside diameter is formed at the inner end thereof.
The inner ring 3 having a second inner raceway 9 formed therein is fit to the stepped part 8. A male threaded portion 10 is formed in the inner end of the hub 2. The top end (the right end in FIG. 4) of the male threaded portion 10 is extended inward beyond the inner end face of the inner ring 3. The inner ring 3 is held between the nut 11 screwed to the male threaded portion 10 and a step surface 12 of the stepped part 8, whereby the inner ring 3 is firmly coupled to a predetermined position on the hub 2. An engaging recess 13 is formed in the outer peripheral surface of the top end of the male threaded portion 10. The nut 11 is fastened by a predetermined torque, and then a part of the nut 11, which is aligned with the engaging recess 13, is radially and inwardly caulked, thereby preventing the loosening of the nut 11.
A first outer raceway 14 and a second outer raceway 15 are formed in the inner peripheral surface of the outer ring 4. The first outer raceway 15 is confronted with the first inner raceway 7. The second outer raceway 15 is confronted with a second inner raceway 9. A plurality of rolling elements 5 are located between the first inner raceway 7 and the first outer raceway 14. A plurality of rolling elements 5 are located between the second inner raceway 9 and the second outer raceway 15. In the illustrated instance, the rolling elements 5 are balls; however, in the case of the wheel-support rolling bearing unit for motor vehicles which is heavy, those may be tapered rollers.
To assemble the wheel-support rolling bearing unit 1 into the motor vehicle, the outer ring 4 is fixed to a suspension device with a second flange 16 formed on the outer peripheral surface of the outer ring 4, and the wheel is fixed to the first flange 6. As a result, the wheel is rotatably supported on the suspension device.
In Japanese Patent Unexamined Publication No. Hei.11-129703, there is described a wheel-support rolling bearing unit 1a as shown in FIGS. 5 to 7. The wheel-support rolling bearing unit 1a as a second conventional technique includes a hub 2a, an inner ring 3, an outer ring 4, and a plurality of rolling elements 5. A first flange 6 for supporting the wheel is formed at a portion of the hub 2a which is closer to the outer end of the outer peripheral surface thereof. A first inner raceway 7 is formed in the outer surf ace of a middle portion of the hub 2a. A stepped part 8 having the reduced outside diameter is formed at the inner end thereof.
A cylindrical portion 18, which is used for forming a caulking portion 17 to fix the inner ring 3, is formed in the inner end of the hub 2a. A thickness of the cylindrical portion 18 becomes small toward its top end (the right end in FIG. 7) before the cylindrical portion 18 shown in FIG. 7 is expanded, for caulking, outward in the diameter directions. Accordingly, a tapered hole 19 is formed in the inner end face of the hub 2a. The inside diameter of the tapered hole becomes small toward a recess.
To expand, for caulking, the top end of the cylindrical portion 18 in order to fasten the inner ring 3 to the inner end of the hub 2a, a force piston 20 is forcibly pressed against the top end of the cylindrical portion 18 in a state that the hub 2a is fixed so as not to shift in the axial direction, as shown in FIG. 6. An outward curved portion 21, shaped like a truncated cone, which may be thrust into the cylindrical portion 18, is formed in the central portion of the end surface (left end surface in FIG. 6) of the force piston 20. An inward curved portion or recess 22 is formed surrounding the outward curved portion 21. A cross section of the recess 22 is configured to have a complex curved surface whose radius of curvature becomes small toward the outside so that a cross section of the caulking portion 17, which is formed by plastically forming the top end of the cylindrical portion 18 by the recess 22, gradually reduces in size from the base end to the top end, and abruptly reduces at the top end.
By pressing the force piston 20 having the outward curved portion 13 and the recess 22, which are shaped and have dimensions as mentioned above, against the top end of the cylindrical portion 18, the top end of the cylindrical portion 18 is forcibly expanded outward in the diameter directions for caulking, to thereby form a caulking portion 17. The inner ring 3 is firmly held between the caulking portion 17 and a step surface 12 of the stepped part 8 formed at the inner end of the hub 2a, whereby the inner ring 3 is fastened to the hub 2a. 
For a work to form the caulking portion 17 by plastically deforming (expanding for caulking) the cylindrical portion 18, it is preferable to use a swing press 23 as shown in FIG. 8. The swing press 23 is made up of a force piston 20, a holding tool 24 and a holder 25. In forming the caulking portion 17 by expanding for caulking the cylindrical portion 18, the force piston 20 is displaced in a swing manner while pushing upward the hub 2a with the aid of the holder 25. In a state that the center axes of the force piston 20 and the hub 2a are inclined at an angle xcex8 with respect to each other, the force piston 20 is turned about the center axis of the hub 2a, while being in contact with each other. In forming the caulking portion 17 by the swing press as just-mentioned, a part of the circumference of the force piston 20 is pressed against the cylindrical portion 18, so that the caulking expanding work of the caulking portion 17 continuously progresses in part in the circumferential direction. For this reason, when comparing with the forming of the caulking portion 17 by ordinary forging process, a load acting on the cylindrical portion 18 may be reduced during the forging process. The holding tool 24 prevents the inner ring 3 and the hub 2a from moving in the radial direction during the caulking expanding work of the caulking portion 17 by the force piston 20.
In the conventional wheel-support rolling bearing unit thus constructed, it is difficult to apply a pre-load to the rolling elements 5 and to render normal a fitting state of the inner ring 3 to the hub 2a. In recent years, a measure in which the pre-load is applied to the rolling elements 5 has been employed in order to increase a rigidity of the wheel-support rolling bearing unit. The related component parts are dimensioned so that the pre-loading has an appropriate value in a state that the outer end face 28 of the inner ring 3 is in contact with the step surface 12 formed on the outer peripheral surface of the hub 2a. 
In a state that the outer end face 28 of the inner ring 3 is brought into contact with the step surface 12 to pre-loading the rolling elements 5, a thrust load acts on the inner ring 3 based on the pre-loading, in the direction causing the inner ring 3 to move apart from the step surface 12. In the case of the conventional structure, the inner ring 3 is axially displaced by such a thrust load before the caulking portion 17 is formed at the inner end of the hub 2a, and as a result, the outer end face 28 is separated from the step surface 12.
If the work to form the caulking portion 17 starts from a state that the outer end face 28 of the inner ring 3 is separated from the step surface 12, the inner ring 3 is pressed against the step surface 12, while being scraped off. Specifically, with the formation of the caulking portion 17, the inner ring 3 is pushed onto the stepped part 8. At this time, the direction of the force pressing the inner ring 3 is slanted with respect to the directions of the axes of the inner ring 3 and the hub 2a. Accordingly, the circumferential edge of the opening of the outer end of the inner ring 3 is pushed onto the stepped part 8 while forcibly rubbing against the outer peripheral surface of the stepped part 8 (biting into the outer peripheral surface of the stepped part 8).
As a result, a small depression is formed in the outer peripheral surface of the stepped part 8. This possibly leads to degradation of the roundness deviation of the second inner raceway 9, which is formed in the outer peripheral surface of the inner ring 3, and an inclination (small as it is) of the center axis of the inner ring 3 with respect to the center axis of the hub 2a. The degradation of the circularity deviation and the center axis inclination are both undesirable since those will cause the vibration during the operation of the wheel-support rolling bearing unit to increase and the unit performances to deteriorate.
It is an object of the present invention to provide a wheel-support rolling bearing unit and method of manufacturing the same, which can prevent the outer peripheral surface of the stepped part from being depressed and the center axis of the inner ring from being inclined.
According to one aspect of the invention, there is provided a wheel-support rolling bearing unit comprising: a hub having a first flange formed on the outer peripheral surface of one end thereof, and a first inner raceway formed in the outer peripheral surface of a middle portion thereof with an inner ring integrally or separately formed; a stepped part formed at the other end of the hub, the inside diameter of the stepped part being smaller than of a portion of the hub where the first inner raceway is formed; an inner ring having a second inner raceway formed in the outer peripheral surface thereof, the inner ring being fit to the stepped part; an outer ring having first and second outer raceways formed in the inner peripheral surface, the first outer raceway being confronted with the first inner raceway and the second outer raceway being confronted with the second inner raceway; and a plurality of rolling elements being located between the first inner raceway and the first outer raceway and a plurality of rolling elements being located between the second inner raceway and the second outer raceway; wherein by a caulking portion, which is formed by radially and outwardly expanding for caulking a cylindrical portion formed at a portion of the other end of the hub, which is protruded beyond at least the inner ring fit to the stepped part, the inner ring fit to the stepped part is held down against the step surface of the stepped part, and firmly coupled to the hub.
In the wheel-support rolling bearing unit, a pre-load is applied to the rolling elements. The inner ring is fit to the stepped part by interference fitting (in other words, fittingly inserting with interference). A static friction force which acts on the inner ring by the interference fitting is larger than an axial load acting on the inner ring by the pre-load applied to the rolling elements.
According to another aspect of the invention, there is provided a method for manufacturing the wheel-support rolling bearing unit defined as mentioned above, comprising the steps of: fitting the inner ring to the stepped part of the hub by interference fitting; butting the end face of the inner ring against the step surface of the stepped part of the hub, to thereby applying a pre-load to the rolling elements; forming a caulking portion at the stepped part by radially and outwardly expanding the cylindrical portion formed at the other end of the hub in a state that the inner ring is fixed to the stepped part by the static friction force based on the interference fitting in a state that the end face of the inner ring is butted against and fixed to the step surface of the stepped part of the hub.
The object of the present invention can be achieved by a wheel-support rolling bearing unit comprising an outer ring, an inner ring, a hub, a plurality of first and second rolling elements. The outer ring has first and second outer raceways formed on its inner peripheral surface. The hub has a flange formed on the outer peripheral surface of one end thereof, the hub having a first inner raceway which is disposed on its middle portion in an axial direction thereof and is confronted with the first outer raceway of the outer ring, the hub having a stepped part and a cylindrical portion disposed at the other end of the hub, a step surface being defined on the hub by the stepped part. The inner ring is fit to the stepped part of the hub with interference while the end surface of the inner ring is brought in contact with the step surface of the hub, the inner ring having a second inner raceway which is formed on its outer peripheral surface and is confronted with the second outer raceway of the outer ring. The plurality of first rolling elements are located between the first inner raceway and the first outer raceway; and the plurality of second rolling elements are located between the second inner raceway and the second outer raceway. In the construction, a caulking portion, which is formed by plastically deforming the cylindrical portion, is protruded beyond at least the inner ring thus fit to the hub. In the above-mentioned wheel-support rolling bearing unit, the inside diameter of the stepped part is smaller than that of a portion of the hub where the first inner raceway is formed. Further, in the above-mentioned wheel-support rolling bearing unit, a pre-load is applied to the first and second rolling elements by bringing the end surface of the inner ring in contact with the step surface of the stepped part prior to the caulking, and a static friction force which acts on the inner ring by the interference fitting is larger than an axial load acting on the inner ring by the pre-load applied to the rolling elements.
In the above-mentioned wheel-support rolling bearing unit according to the present invention, it is also advantageous that the hub comprises:
a main body integrally formed with the flange portion; and
a separate inner ring which has the first inner raceway and is fit to the main body, the step surface being formed on an end surface of the separate inner ring.
Moreover, the wheel-support rolling bearing unit according to the present invention, it is more preferable that a stress, which is caused in the circumferential direction in the inner ring by the interference fitting, is smaller than the result of subtracting a stress, which acts on the inner ring in the circumferential direction when the caulking portion is formed, from a tolerable stress acting on the inner ring in the circumferential direction.
The above-mentioned object can also be achieved by a wheel-support rolling bearing unit, according to the present invention comprising an outer ring, an inner ring, a hub, a plurality of first and second rolling elements. The outer ring has first and second outer raceways formed on its inner peripheral surface. The hub has a flange formed on the outer peripheral surface of one end thereof, the hub having a first inner raceway which is disposed on its middle portion in an axial direction thereof and is confronted with the first outer raceway of the outer ring, the hub having an end portion disposed at the other end of the hub. The inner ring is fit to the end portion with interference, the inner ring having a second inner raceway which is formed on its outer peripheral surface and is confronted with the second outer raceway of the outer ring. The plurality of first rolling elements are located between the first inner raceway and the first outer raceway, and the plurality of second rolling elements are located between the second inner raceway and the second outer raceway. In the roller bearing unit, a caulking portion, which is formed by plastically deforming a portion of the end portion of the hub, is protruded beyond at least the inner ring thus fit to the hub, and a stress, which is caused in the circumferential direction in the inner ring by the interference fitting, is smaller than the result of subtracting a stress, which acts on the inner ring in the circumferential direction when the caulking portion is formed, from a tolerable stress acting on the inner ring in the circumferential direction.
In the wheel-support rolling bearing unit, it is advantageous that the hub comprises a main body integrally formed with the flange portion; and a separate inner ring which has the first inner raceway and is fit to the main body.
The above-mentioned object can also be attained by a method for manufacturing a wheel-support rolling bearing unit comprising:
an outer ring having first and second outer raceways formed on its inner peripheral surface;
a hub having a flange formed on the outer peripheral surface of one end thereof, the hub having a first inner raceway which is disposed on its middle portion in an axial direction thereof and is confronted with the first outer raceway of the outer ring, the hub having a stepped part and a cylindrical portion disposed at the other end of the hub, a step surface being defined on the hub by the stepped part;
an inner ring provided at the other end of the hub, the inner ring having a second inner raceway which is formed on its outer peripheral surface and is confronted with the second outer raceway of the outer ring;
a plurality of first rolling elements located between the first inner raceway and the first outer raceway; and
a plurality of second rolling elements located between the second inner raceway and the second outer raceway;
the method comprising:
fitting the inner ring to the stepped part of the hub by interference fitting while bringing an end surface of the inner ring into contact with the step surface of the hub, to thereby applying a pre-load to the rolling elements;
forming a caulking portion by plastically deforming the cylindrical portion, wherein the caulking portion is protruded beyond at least the inner ring thus fit to the hub.
In the method according to the present invention, it is advantageous that a static friction force which acts on the inner ring by the interference fitting is larger than an axial load acting on the inner ring by the pre-load applied to the rolling elements.
Further, in the method according to the present invention, it is preferable that a stress, which is caused in the circumferential direction in the inner ring by the interference fitting, is smaller than the result of subtracting a stress, which acts on the inner ring in the circumferential direction when the caulking portion is formed, from a tolerable stress acting on the inner ring in the circumferential direction.
According to the wheel-support rolling bearing unit and method of manufacturing the same, the inner ring is not displaced to the hub when the caulking portion is formed. Specifically, the inner ring is fit to the stepped part of the hub prior to the formation of the caulking portion. The end face of the inner ring is left as it is in contact with the stepped part independently of the thrust load based on the pre-load in a state that the respective rolling elements are pre-loaded. Accordingly, even if with progress of the caulking portion forming work, the force slanted in direction with respect to the center axes of the inner ring and the hub act onto the inner ring, it never happens that the circumferential edge of the opening of the outer end of the inner ring forcibly rubs against the outer peripheral surface of the stepped part 8 and that the center axis of the inner ring slants with respect to the center axis of the hub.